Lot of stuff that makes new ST much more usable and engaging for me:
Like button for posts
Ability to either reply generally to a thread or to a specific post
All replies to a particular post are aggregated and shown together in a little dropdown (this might be desktop only?)
You can insert images by just pasting
Large threads are much easier to navigate – nice slider that lets you easily jump to a particular post # or date, no more guessing pages to find a date
Support for markdown syntax, much easier to do basic formatting like italics, bold, monospaced, links, bullets, tables, etc without clicking tiny buttons
Lots of user settings available to fine-tune the forum’s behavior to suit your preference
New forum actually accurately keeps track of what I have and have not viewed. Old forum would drop me into the middle of threads I’d never read before.
Search actually works and is not completely insane
Reply notification emails actually contain the damn reply. And have a working link in them rather than plaintext URL I need to copy/paste into the browser.
I can curate (mute) categories that I’m not interested in.
I can still get a list of everything new (or all latest) posts even though there are now categories/subforums for things… Like the old forum… Except that it excludes things I’ve curated out.